This invention relates to fuel control systems for delivering fuel flow to gas turbine engines, and relates more particularly to an improved method and apparatus for controlling change in speed of the engine.
Performance of a gas turbine engine, including its acceleration characteristics, are limited by the onset of compressor instability or stall. Accordingly, fuel control systems for a gas turbine engine are devised to, in one way or another, control fuel flow to avoid the region of compressor instability. For fastest engine acceleration during starting, on the other hand, it has generally been proposed to control fuel flow to the engine such that engine performance closely approaches but does not encounter the region of instability or stall. In this manner, it has been theorized that power is developed most rapidly by the engine and maximum acceleration thus results. To accomplish this result, the majority of prior art fuel control systems are primarily concerned with sensing the proper engine performance parameters which indicate the engine is operating at its near maximum performance before encountering stall, and accordingly scheduling fuel flow in response to these sensed parameters. Thus, sensing temperature and pressure at various locations in the gas flow path through the engine, then scheduling fuel flow in relation to these various sensed temperatures, pressures and other factors, has been a conventional approach to scheduling fuel flow to the engine during its acceleration. By attempting to accelerate the engine closely to its region of instability, such prior art fuel control systems also were well suited to avoid engine operation near its required-to-run line during acceleration to minimize the possibility of "hung" starts. Conversely to the instability or stall characteristics of a particular engine, the required-to-run line of the engine dictates the minimum amount of fuel flow required to the engine in order to overcome the inertia thereof, load imposed thereon, and the like in order to maintain a given speed. Fuel flow less than that dictated by the required-to-run line causes deceleration of the engine to a slower speed, and to accelerate the engine, obviously, the fuel flow to the engine must be above the required-to-run line.
A serious drawback for fuel control systems operating to accelerate a gas turbine engine relates to the inability to obtain sufficiently accurate measurements of the various parameters of pressure, temperature, etc. utilized in controlling fuel flow. Such sensing devices have proved to be somewhat unreliable due to slow response time and ultimate deterioration from the extreme environmental conditions to which they may be exposed. Beyond this, it is also recognized that the sensing of such various engine parameters only gives a general "estimate" of how close the engine may be running to its instability region for a given set of external conditions. Onset of compressor instability or engine stall varies greatly dependent upon external ambient conditions such as temperature and pressure of the ambient airflow being received by the engine, as well as even the temperature of the fuel flow being delivered to the engine. Such change in environmental conditions which markedly affect engine performance characteristics are highly pronounced in aircraft applications of gas turbine engines where the ambient conditions can vary drastically. Accordingly, fuel control systems have become more and more sophisticated to compensate for various changes in external conditions such that conditions of instability are avoided, yet while assuring the engine can still be maintained above its required-to-run line. For instance, the fuel control system must take into account sufficient variations in engine performance characteristics so as to avoid "hot starts" which are more likely to occur at high altitude restarts of an aircraft mounted engine. "Hot starts" are a result of the engine running near its stall line by delivering a high volume fuel flow to the engine combustion chamber such that a relatively high temperature at the discharge of the engine turbine and downstream of the combustion chamber, as well as high temperature exhaust from the engine results. Excessive temperature on the turbine and exhaust components created by such "hot starts" can cause engine failure due to overheating of these downstream components. Exemplary disclosures of prior art systems may be found in U.S. Pat. Nos. 3,011,310; 3,043,367; 3,085,619; 3,139,892; 3,399,527.